SANTA CLARA, Calif. — Within the first 12 minutes of game time of the first preseason game of 2018, the San Francisco 49ers saw four starters go down with injury. None of those starters was named Jimmy Garoppolo, but it was nevertheless an inauspicious start for the first 49ers team in four years to have lofty preseason expectations.

Thursday night’s tilt against the Dallas Cowboys, went to San Francisco, 24-21 on a two-minute drive, capped by a seven-yard scoring dart from QB Nick Mullins to Richie James.

While the first preseason win against the Cowboys in their last three tries was certainly a nice way to send the fans home, the 49ers should be concerned about the state of their defense after losing three of their top defenders to injury, among six players overall.

“It’s what scares you about training camp, it’s what scares you about preseason; you just want the game to end,” said head coach Kyle Shanahan, who lost linebackers Malcolm Smith and Eli Harold (lower leg), as well as defensive lineman Solomon Thomas.

“It’s never good to see,” Garoppolo said. “It’s tough to see that in preseason.”

With about 7:30 to go in the first quarter, Smith exited with a hamstring injury. He did not return. With Reuben Foster suspended for the first two regular-season games, and with both Brock Coyle and Fred Warner sidelined before the game, that would mean San Francisco’s top two inside linebackers are Korey Toomer and Elijah Lee.

Toomer wasn’t great. With just over six minutes to go in the second quarter, Toomer couldn’t hold on to third-string back Bo Scarbrough on a 26-yard run after getting good position on him at the point of attack.

After missing a deep pass by Garoppolo up the seam — an overthrow — tight end George Kittle, too, went down, and was examined on the sideline with 5:45 to go in the first. About a minute and a half of game time later, he went into the locker room with his right arm pinned tightly to his side. The official diagnosis was a right shoulder injury.

Moments later, Thomas — who teamed up with DeForest Buckner for a first-quarter sack — went down after colliding with Cassius Marsh and remained on the field for four minutes, before being helped off by the 49ers medical staff. He was diagnosed officially with a head injury.

With 3:18 to go, running back Matt Breida went for a cut block on defensive tackle Antwaun Woods, and looked to jam his shoulder. He walked off the field under his own power, escorted by 49ers medical staff. That injury didn’t look very serious.

As if that wasn’t enough, at 14:51 in the second quarter, backup right tackle Gary Gilliam went down with a head injury at the 30-yard line on a run play.He was attended on the field by medical staff, but left the field under his own power.

Gilliam and Thomas had concussion symptoms, Shanahan said. Kittle and Breida’s shoulder injuries could keep them out “for some time,” and that means they’re likely out for next week against Houston.

Mixed bag. We saw the first-round offensive tackle out of Notre Dame for a full quarter — more than any starter — and while McGlinchey did get beat on some run plays, there was one play with 3:18 left in the first quarter where he tossed his defender around like a ragdoll. There is clearly a ton of upside — as there should be, with a first-rounder — and seeing McGlinchey at full speed solidified that for me.

His pass sets are very clean, and I like his hip sink and pad level, but there was a run play or two where I saw him get out over his skis.

That said, there’s only so much you can tell from 14 snaps (unofficial), but given the fact that McGlinchey follows Joe Staley around like a puppy dog (it’s hard not to hear “Best Friend” playing on the soundtrack), all that raw power and strength is going to be polished in short order. Once he learns how to use his size, he’s going to be terrifying.

2. Can Joe Williams make a case for the 49ers to keep four RBs on the roster?

Overall, not inspiring, but he did have a 15-yard pinball run early in the fourth quarter, for whatever that’s worth in the first preseason game. His only first-half carry against a unit comprised mainly of second-teamers saw him dropped for a loss.

Williams finished with 27 yards on 11 carries.

We didn’t see anything like the regular explosive plays we get just about every practice. He did split out wide in an empty set during the second quarter, but was only targeted once in the first half, on a short wheel route for six yards in the second quarter.

After a pass interference call got the 49ers into the red zone, he rushed six yards to get San Francisco inside the two, and then plunged in for the 49ers first score on a tough inside run.

Not sexy, but it was what the 49ers needed at the time, and there’s certainly a need for that in today’s game. However, you don’t need that from the former fourth-round pick out of Utah, not when you have two guys who already can get that done in Jerick McKinnon and Matt Breida. Williams is going to have to find a way to distinguish himself if he wants to be that third back.

Jeremy McNichols, the fifth running back to enter, had a very nice series leading to the 49ers’ second touchdown of the night, rushing three times for 15 yards in the fourth quarter. He may wind up entering sooner next week in Houston against the Texans. Raheem Mostert had the best day of the San Francisco backs, rushing for 57 yards on eight carries.

3. Will Dante Pettis continue to be timid over the middle of the field?

Didn’t get to see much of Pettis over the middle, but he did have one of the game’s highlights: a 53-yard grab against the Dallas second-string secondary with 13 minutes to go in the third quarter. He’s going to be able to get behind just about every first-string secondary, so that matchup just wasn’t fair.

He did, however, have his rookie moments. There was a big miscommunication with 13:28 left in the first half, where Pettis went for a comeback, but C.J. Beathard looked for a post, overthrowing Pettis by a wide margin. That’s what the preseason is for.

We did see Kendrick Bourne snag a leaping 18-yard grab from Beathard up the seam several plays earlier, at the start of the quarter. He made the catch with Chidobe Awuzie all over his back.

He’s made a few big plays like that already in practice, and he may be the sleeper pick at wide receiver this year for San Francisco.

4. What does San Francisco do at right guard?

The 49ers started Mike Person at right guard, and ran some decent runs behind him, but it wasn’t anything to write home about. No one really distinguished themselves at the position while both sets of starters were still in.

Person, also the backup center, took snaps at right guard to start the second quarter, as well, while Beathard took snaps from Erik Magnusson.

We’re not going to get much clarity at the position until maybe the third preseason game, if Joshua Garnett can manage to return, but that looks increasingly doubtful. Person may be the best bet, given his experience, until Jonathan Cooper can get healthy.

5. Which young DBs can make set themselves apart from the pack?

Couldn’t really tell much, but we did see rookie defensive back D.J. Reed move from slot corner to free safety, which we’d seen a bit of this week. He acquitted himself decently, but it’s tough until we get to see tape and talk with the staff after they see the tape.

Overall, the secondary looked fairly soft, but that’s what’s going to happen when you have absences from the amount of front-line linebackers and defensive linemen San Francisco was working with.

There was a huge blown coverage on Antone Exum Jr. and just about everyone else in the secondary on a 31-yard pass to fourth-string wide receiver Marchie Murdock with 7:59 to go in the third.

If we’re grading this unit, it’s incomplete.

Notebook:

Beathard started 1-of-6, but completed nine of his next 14 passes, finishing with 181 yards.

Garopolo only threw six passes, completing three for 34 yards, including a 15-yard out route to Marquise Goodwin that went for 18. He went for Goodwin the next two throws, but both fell incomplete.

*****

Defensive tackle Julian Taylor played wire-to-wire and recorded a fourth-quarter sack.

“I knew we were going to play him a lot,” Shanahan said.

*****

Foster played into the third series for the 49ers defense, likely because the 49ers want him to get as many reps as possible while he is still allowed to, before missing the first two games of the regular season with a suspension.

He rewarded the 49ers with a leaping breakup of a pass intended for Tavon Austin with 34 seconds left in the opening quarter. Two plays later, Antone Exum locked down Allen Hurns along the sideline to stifle a third-and-seven pass by backup Cooper Rush and force a punt.

*****

After Aldrick Robinson caught a 33-yard pass at the knees from Beathard, avoiding a pick by Duke Thomas, the 49ers surrendered a turnover, an 18-yard pick by linebacker Joe Thomas on a pass intended for Aaron Burbridge.

Cole Hickutini made a diving catch over the middle on a low ball by Betheard with 1:57 to go in the first half. He’s fairly solid in the third tight end for the 49ers behind Garrett Celek and Kittle.

*****

James took a screen pass from Beathard for 21 yards, side-stepping safety Kavon Frazier late in the first half.

“There’s a lot of upside to him,” Beathard said. “He’s a good target.”

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